Nike Zoom Air II clicking noise

I have a pair of Nike Zoom Air II that's fairly new (sole not worn yet, maybe 20-30 hours of play), and the right heel started making this clicking sound when I walk, on every step, around the heel area. There's no visible crack or damage on the sole at all. The inside looks fine, too.

Has anyone experience something similar on their shoes? Maybe the glue is undone a little bit somewhere? The shoe still seems to hold up fine. It's just annoying to have a little clicking sound when you walk around.

The same thing happened to me with 2 pairs of Air Edge X-trainers, even though I only used them as casual kicks. Ditto with one of my friend's Air "???" (mid-line shoe bought at Sports Authority). The noise is fairly subtle, but drove me insane. Admittedly, it is a short drive

Well, the clicking noise came back, of course, louder than ever, and is starting to drive me insane, too.

However, I don't feel much of a difference in the cushioning/support. At least not yet. If there were no clicking sound, I probably wouldn't have noticed anything wrong. It's not like I'm now walking around lopsided or anything like that.

So I guess the question is whether it's still safe to wear it (if I can ignore the clicking sound)?